The recent announcement from ESO that they have discovered a candidate exoplanet orbiting Proxima Centauri – thus confirming weeks of speculation – has certainly been exciting news! Not only is this latest discovery the closest extra-solar planet to our own solar system, ESO has also indicated that it is rocky, similar in size and mass to Earth, and orbiting the habitable zone of the star.
However, in the midst of this news, there has been some controversy regarding certain labels. For example, when a planet like Proxima b is described as “terrestrial”, “habitable” and / or “terrestrial”, there are naturally questions about what this really means. For each term there are particular implications, which in turn require clarification.
For starters, calling a planet “Earth-like” usually means that it is similar in composition to Earth. This is where the term “terrestrial” really comes in, as it refers to a rocky planet made up mostly of silicate rocks and metals differentiated between a metallic core and a silicate mantle and crust.
This applies to all planets in the inner solar system and is often used to differentiate rocky exoplanets from gas giants. This is important in the context of the hunt for exoplanets, as the majority of 4,696 exoplanet candidates – of which 3,374 have been confirmed (as of August 18, 2016) – have been gas giants.
What does it do not means, at least not automatically, that the planet is habitable in the same way as the Earth. Merely being terrestrial in nature is not an indication that the planet has a suitable atmosphere or a climate warm enough to support the existence of liquid water or microbial life on its surface.
Additionally, Earth-like generally implies that a planet will be similar in mass and size to Earth. But this is not the same as the composition, as many exoplanets discovered have been labeled as “the size of the Earth” or “super-Earths” – that is, planets with about 10 times the mass. of the Earth – only on the basis of their mass.
This term also distinguishes a candidate exoplanet from those with 15 to 17 masses (which are often referred to as “the size of Neptune”) and those with masses similar or several times greater than that of Jupiter (i.e. to say Super-jupiter). In all of these cases, size and mass are the qualifiers, not composition.
Ergo, finding a planet that is larger in size and mass than Earth, but significantly less than that of a gas giant, does not mean that it is terrestrial. In fact, some scientists have recommended that the term “mini-Neptune”Be used to describe planets more massive than Earth, but not necessarily composed of minerals and silicate metals.
And size and mass estimates aren’t exactly measures of whether a planet is “habitable” or not. This term is particularly sticky when it comes to exoplanets. When scientists associate this word with extra-solar planets like Proxima b, Gliese 667 Cc, Kepler-452b, they generally refer to the fact that the planet exists in the “habitable zone” of its mother star (aka the Goldilocks zone).
This term describes the region around a star where a planet will experience average surface temperatures allowing liquid water to exist on its surface. For planets that orbit too close to their star, they will experience intense heat that will transform surface water into hydrogen and oxygen – the former escaping into space, the latter combining with carbon to form CO² .
This is what scientists think happened to Venus, where thick clouds of CO² and water vapor set off an uncontrollable greenhouse effect. It turned Venus away from a world that once had oceans in the hellish environment we know today, where temperatures are hot enough to melt lead, atmospheric density so off the charts and sulfuric acid rains from its thick clouds.
For planets that orbit beyond a star’s habitable zone, water ice will become frozen solid, and the only liquid water will likely be in underground reservoirs (this is the case on Mars). As such, finding planets that are fair in terms of average surface temperature is intrinsic to the “handy fruit” approach of the search for life in our Universe.
But of course, just because a planet is hot enough to have water on its surface doesn’t mean that life can thrive there. As our own solar system beautifully demonstrates, a planet can have the conditions necessary for life, but still become a sterile environment because it does not have a protective magnetosphere.
This is what scientists believe happened to Mars. Located in the Goldilocks area of our Sun (albeit on the outer edge of it), Mars would once have had an atmosphere and liquid water on its surface. But today, the atmospheric pressure on the surface of Mars is only 1% of that of the Earth, and the surface is dry, cold and devoid of life.
The reason has been determined to be that Mars lost its magnetosphere 4.2 billion years ago. According to NASA’s MAVEN mission, Mars’ atmosphere was slowly destroyed over the next 500 million years by the solar wind. The little atmosphere it had left was not enough to retain heat, and its surface water evaporated.
Likewise, planets devoid of protective magnetospheres are also subjected to an intense level of radiation on their surface. On the Martian surface, the average dose of radiation is about 0.67 millisieverts (mSv) per day, or about one-fifth of what people are exposed to here on Earth over the course of a year.
Similar situations can be expected on extra-solar planets where there is no magnetosphere. Essentially, the Earth is fortunate not only to orbit somewhere comfortable enough around our Sun, but also that its core differentiates between a solid inner core and a rotating liquid outer core. This rotation, it is believed, is responsible for creating a dynamo effect which in turn creates the Earth’s magnetic field.
However, again using our own solar system as a model, we find that magnetic fields are not entirely rare. While Earth is the only terrestrial planet in our solar system (all gas giants have strong fields), Jupiter’s moon, Ganymede, also has its own magnetosphere.
Likewise, there are orbital parameters to consider. For example, a planet of similar size, mass, and composition could still have a very different climate than Earth because of its orbit. On the one hand, it can be tidal locked with its star, which would mean that one side is facing it all the time and is therefore much hotter.
On the other hand, it can have a slow rotational speed and fast orbital speed, which means it only experiences a few rotations per orbit (as is the case with Mercury). Last, but not least, its distance from its respective star could mean that it receives much more radiation than Earth, whether or not it has a magnetosphere.
It is believed to be the case with Proxima Centauri b, which orbits its red dwarf star at a distance of 7 million km (4.35 million mi) – just 5% of the distance from Earth to the Sun. It also orbits Proxima Centauri with an orbital period of 11 days, and has either synchronous rotation or 3: 2 orbital resonance (i.e. three rotations for two orbits).
For this reason, the climate is likely to be very different from that of Earth, with water confined either on its side facing the sun (in the case of synchronous rotation) or in its tropical zone ( in the case of a 3: 2 resonance). In addition, the radiation it receives from its red dwarf star would be significantly higher than what we are used to here on Earth.
So what exactly does “like the earth” mean? The short answer is, it can mean a lot. And in that regard, it’s a pretty dubious term. If Earth-like can mean similarities in mass, size, composition, and can allude to the planet orbiting within its star’s habitable zone – but not necessarily all of the above – then it’s not a very reliable term.
Ultimately, the only way to keep things clear would be to describe a planet as “Earth-like” if in fact it has similarities in size, mass, and composition, all at the same time. The word “terrestrial” can certainly be substituted at a pinch, but only when the composition of the planet is known with a reasonable degree of certainty (and not just its size and mass).
And words like “habitable” should probably only be used when chaperoned by words like “potentially”. After all, being in a star’s habitable zone certainly means there is the potential for life. But that does not necessarily imply that life may have emerged there, or that humans may one day live there.
And should these words apply to Proxima b? Maybe, but you have to consider the fact that ESO announced the detection of an exoplanet using the Radial speed method. Until confirmed by direct detection methods, it remains a candidate (unconfirmed) exoplanet.
But even these simple steps would probably not be enough to erase any ambiguity or controversy. Ultimately, the hunt for planets – like all aspects of space exploration and science – is a matter of division. And new findings always have a way of attracting criticism and disagreement from several parties at once.
And you thought Pluto’s classification was confusing things! Well, Pluto has nothing on the exoplanet database! So be prepared for many years of classification debate and controversy!
Further reading: NASA Exoplanets Archives